Download Computer Interfaces - Experimental Techniques - Lecture Slides and more Slides Electrical Engineering in PDF only on Docsity!
Computer Interfaces
Serial, Parallel, GPIB, CAMAC, Oh My!
Common Implementations of Interfaces
•^
Parallel port (8 bits per shot)
•^
Serial (RS-232, RS-485)– usually asynchronous
•^
GPIB (IEEE-488) parallel– General Purpose Interface (or Instrument) Bus– originally HPIB; Hewlett Packard
•^
DAQ card (data acquisition)– like national instruments A/D, D/A, digital I/O
•^
CAMAC– Computer Automated Measurement And Control
•^
VME bus / VXI bus– modern CAMAC-like bus
A quick note on hexadecimal
decimal value
binary value
hex value
0
0000
0
1
0001
1
2
0010
2
3
0011
3
4
0100
4
5
0101
5
6
0110
6
7
0111
7
8
1000
8
9
1001
9
10
1010
a
11
1011
b
12
1100
c
13
1101
d
14
1110
e
15
1111
f
Hexadecimal, continued
•^
Once it is easy for you to recognize four bits at atime, 8 bits is trivial:– 01100001 is 0x61– 10011111 is 0x9f
•^
Can be handy because the ASCII code is builtaround hex:– ‘A’ is 0x41, ‘B’ is 0x42, …, ‘Z’ is 0x5a– ‘a’ is 0x61, ‘b’ is 0x62, …, ‘z’ is 0x7a– ‘^A’ (control-A) is 0x01, ‘^B’ is 0x02, ‘^Z’ is 0x1A– ‘0’ is 0x30, ‘9’ is 0x
Docsity.com
Exchanging Data
•^
Parallel: Fast and expensive–
devices A, B simple, but cablingharder– strobe alerts to “data valid” state
Device A
bit 0bit 1bit 2bit 3bit 4bit 5bit 6bit 7
Device B
strobe
•^
Serial: Slow and cheap–
but devices A and mustconvert between serial/parallel
Device A
Device B
data
slide courtesy E. Michelsen
The Parallel Port
•^
Primarily a printer port on the PC– goes by name LPTx: line printer– usually LPT
•^
8 data bits– with strobe to signal valid data– can be fast (1 Mbit/sec)
•^
Other control and status bits for (printer) communication
data valid
data held static for some interval
see http://www.beyondlogic.org/index.html#PARALLEL
Parallel Port Pinout
Parallel Port Access
•^
Most PCs have a DB-25 femaleconnector for the parallel port
-^
Usually at memory address 0x
-^
Windows 98 and before were easyto talk to–
but after this, a hardware-abstraction layer (HAL) whichmakes access more difficult– one option is to fool computer intothinking you’re talking to a normalLPT (printer) device– involves tying pins 11 and 12 toground
-^
Straightforward on Linux–
direct access to all pins
serial port
parallel port
Docsity.com
Can We Talk?
•^
If we agree on 4 asynchronous communication parameters:–
Data rate: Speed at which bits are sent, in bits per seconds (bps)– Number of data bits: data bits in each byte; usually 8
-^
old stuff often used 7
-^
Parity: An error detecting method: None, Even, Odd, Mark, Space
-^
Stop bits: number of stop bits on each byte; usually 1.
-^
Rarely 2 or (more rarely) 1.5: just a minimum wait time: can be indefinite start
bit 0
bit 1
bit 2
bit 3
bit 4
bit 5
bit 6
parity
stop 1
idle
idle
9600, 7E
start
bit 0
bit 1
bit 2
bit 3
bit 4
bit 5
bit 6
bit 7
stop
idle
idle
ASCII “A” = 0x
9600, 8N
1 bit @ 9600 bps = 1/
th^
sec
stop 2
slide courtesy E. Michelsen
RS-232: most common implementation
•^
RS-232 is an electrical (physical) specification forcommunication–
idle, or “mark” state is logic 1;
-^
5 to
15 V (usually about
12 V) on transmit
-^
3 to
25 V on receive
–^
“space” state is logic 0;
-^
+5 to +15 V (usually ~12 V) on transmit
-^
+3 to +25 V on receive
-^
the dead zone is from
3 V to +3 V (indeterminate state)
•^
Usually used in asynchronous mode–
so idles at
12; start jumps to +12; stop bit at
–^
since each packet is framed by start/stop bits, you are guaranteeda transition at start
-^
parity (if used) works as follows:
-^
even parity guarantees an even number of ones in the train
-^
odd parity guarantees an odd number of ones in the train
GPIB (IEEE-488)
•^
An 8-bit parallel bus allowing up to 15 devicesconnected to the same computer port– addressing of each machine (either via menu or dip-
switches) determines who’s who
- can daisy-chain connectors, each cable 2 m or less in length
•^
Extensive handshaking controls the bus– computer controls who can talk and who can listen
•^
Many test-and-measurement devices equipped withGPIB– common means of controlling an experiment: positioning
detectors, measuring or setting voltages/currents, etc.
•^
Can be reasonably fast (1 Mbit/sec)
Data Acquisition
•^
A PCI-card for data acquisition is avery handy thing
-^
The one pictured at right (NationalInstruments PCI-6031E) has:–
64 analog inputs, 16 bit– 2 DACs, 16 bit analog outputs– 8 digital input/output– 100,000 samples per second– on-board timers, counters
-^
Breakout box/board recommended
Docsity.com
CAMAC
•^
This somewhat old interface provides a “crate” intowhich one slides modules that perform specific tasks– A/D conversion– time-to-digital converters– pulse generators– charge measurement– amplifiers– delay generators
•^
Frequently used in timing experiments, like nuclearphysics: catch events in detector, generate signal,measure strength, etc.
•^
Often the modules are highly multiplexed (16channels per card common)
CAMAC crate (above) and inhabitants(right) including two custom modules,two commercial time-to-digital converters(TDCs) and the crate controller (noteinterface cable (50-pin SCSI-2 style)
CAMAC features
•^
16-bit (newer are 24-bit) data words
•^
Full command cycle in 2
s
8 Mbit/sec
•^
Look-At-Me (LAM) interrupts computer when someevent happens
•^
Commands follow N.A.F. sequence: slot number,address, function– so address specific modules by name/position– A and F values perform tasks that are defined by module– A often refers to channel number on multiplexed device– F might indicate a read, a write, a reset, or other action
Example Interface: APOLLO
•^
APOLLO is a lunar ranging apparatus that fires 20laser pulses per second at a selected lunar reflector,measuring the time-of-flight of photons making theround trip
•^
Besides the essential function of data collection andapparatus coordination, we wanted remote operationcapability
•^
We also required strict thermal control
Docsity.com
